Polyester fibers typified by polyethylene terephthalate can be dyed with only a disperse dye or an azoic dye due to their chemical characteristics. For this reason, polyester fibers have a defect of being less likely to provide a sharp and dark hue. As a method for resolving such a defect, there is proposed a method in which polyester is copolymerized with a metal salt of sulfoisophthalic acid in an amount of 2 to 3 mol % (see, e.g., Patent Document 1 or 2).
However, the polyester fiber obtainable with such a method can be dyed only under high temperatures and high pressures. When the polyester fiber is dyed after interknitting or interweaving with a natural fiber, an urethane fiber, or the like, unfavorably, the natural fiber or the urethane fiber is embrittled. When this is tried to be sufficiently dyed under normal pressure at a temperature of around 100° C., a metal salt of sulfoisophthalic acid becomes required to be copolymerized in a large quantity with polyester. However, in this case, it is difficult to increase the degree of polymerization of polyester due to the thickening effect of the melt viscosity by the action of a sulfonate group. Unfavorably, this results in a remarkable reduction of the strength of the polyester fiber obtained by melt spinning using the polyester, and further results in a remarkable deterioration of the spinning operability. In particular, unfavorably, it is difficult to produce a normal pressure cationic dyeable polyester fiber with a single yarn fineness as fine size as 2 dtex or less.
On the other hand, in order to solve such a problem, there is disclosed a technology of copolymerizing cationic dyeable monomers having a small ion binding intermolecular force (see, e.g., Patent Document 3 or 4). As the cationic dyeable monomers having a small ion binding intermolecular force, there are shown tetrabutylphosphonium 5-sulfoisophthalate, and the like. Polyester resulting from copolymerization of the cationic dyeable monomers is unfavorably inferior in thermal stability. Namely, even when in order to impart normal pressure cationic dyeability thereto, the amount of the compounds thereof to be copolymerized is tried to be increased, thermal decomposition proceeds during the polymerization reaction, which makes it difficult to increase the molecular weight. Further, decomposition by heat history during melt spinning is large, unfavorably resulting in weakening of the resultant yarn. Still further, tetrabutylphosphonium 5-sulfoisophthalate to be used is very expensive, unfavorably resulting in a large increase in cost of the resultant normal pressure cationic dyeable polyester.
As a method for solving such a problem, there is proposed a method, as a method which causes less reduction of the light resistance, and imparts normal pressure dyeability, a method in which a dicarboxylic acid of a straight-chain hydrocarbon such as adipic acid or sebacic acid, or a glycol component such as diethylene glycol, neopentyl glycol, or cyclohexanedimethanol, or polyalkylene glycol with a number-average molecular weight of 400 to 1000 is copolymerized to polyester with a metal salt of sulfoisophthalic acid (see, e.g., Patent Documents 5 to 7).
The methods enable cationic dyeing under normal pressure by reducing the glass transition temperature of polyester by the copolymerizable components, and thereby increasing the diffusion speed of a dye into polyester at a temperature of 100° C. or less. However, with any method, the strength of the normal pressure cationic dyeable polyester fiber obtained by melt spinning the resulting polyester is reduced. Therefore, the tear strength of the fabric obtained using the fiber is reduced, and the glass transition temperature of polyester is reduced. Therefore, the heat setting property is deteriorated, so that false twist crimp processability is inferior, unfavorably resulting in hardened texture, a low dyeing fastness, and other problems. Further, the fiber strength is low, and hence the fine size is difficult to achieve. As a result, unfavorably, it is difficult to obtain a fabric having soft texture.
Whereas, there is proposed a composite fiber including polyester obtained by copolymerizing 5-sodium sulfoisophthalate placed at the sheath part, and polyester including a repeating unit of ethylene terephthalate in an amount of 95 mol % or more placed at the core part (see, e.g., Patent Document 7). However, unfavorably, the amount of copolymerization of the sulfoisophthalic acid metal salt component in copolymerized polyester forming the sheath part has a limitation for the same reason as described above, and hence it is difficult to obtain sufficient dyeability; and formation into a composite fiber results in an increase in processing cost in the spinning step, or results in a restriction on the fiber cross sectional shape; and other problems occur. Further, use of a plurality of kinds of sulfoisophthalic acid components in combination is also disclosed (see, e.g., Patent Document 8). However, within the ranges of the conditions disclosed in the document, the melt viscosity of the resulting polyester is high, so that the strength of the resulting polyester fiber is not sufficient.
Further, when a normal pressure cationic dyeable polyester multifilament and a polyester fiber yarn having a larger hot water shrinkability than this are used to form a differential shrinkage polyester combined filament yarn, the following problems occur. Namely, with a conventional normal pressure cationic dyeable polyester, the hot water shrinkage percentage is high. Thus, the yarn is less likely to become a core-sheath structured yarn in which a normal pressure cationic dyeable polyester yarn uniformly forms the sheath. Further, the fiber strength is low, and hence yarn breakage and fluffing are often caused in the combined filament yarn step, which is a large problem in producing a combined filament yarn with good quality. In view of such circumstances, there has been a demand for a normal pressure cationic dyeable polyester fiber capable of solving the foregoing problems.
Alternatively, also when a normal pressure cationic dyeable polyester is formed into a false twisted textured yarn, the following problems occur. Namely, with a conventional normal pressure cationic dyeable polyester, it is not possible to obtain a yarn satisfying the strength and the elongation capable of standing the false twisting step. Thus, there is a large problem of an increase in occurrence of yarn breakage or fluffing. In view of such circumstances, there has been a demand for a normal pressure cationic dyeable polyester fiber and a normal pressure cationic dyeable polyester false twisted textured yarn each of which has a fine size, and satisfactory strength and elongation, and the like, and thereby is capable of solving the foregoing problems.    [Patent Document 1] JP-B-34-10497    [Patent Document 2] JP-A-62-89725    [Patent Document 3] JP-A-1-162822    [Patent Document 4] JP-A-2006-176628    [Patent Document 5] JP-A-2002-284863    [Patent Document 6] JP-A-2006-200064    [Patent Document 7] JP-A-7-126920    [Patent Document 8] JP-A-1-172425